1. Prior Art Relating to the Disclosure
A great deal of work has been carried out to increase the protection period of biologically active material such as pesticides which have relatively short lives while, at the same time, diminishing the residual waste of such pesticides which are the major cause of serious pollution problems and damage to wildlife. Systems have been developed which utilize the concept of "controlled release" to release the pesticide at a controlled rate effective to control the pest while minimizing the undesired contamination of water sources with subsequent damage to aquatic and wildlife. One method of accomplishing controlled release is by providing solid solutions of pesticides in polymers. U.S. Pat. No. 3,076,744 discloses a urea-formaldehyde resin incorporating up to 10 percent of an organic insecticide and an edible material attractive to insects. U.S. Pat. No. 3,269,900 discloses an intimate admixture of a polyurethane foam with a nonvolatile organic pesticide, the polyurethane foam forming a protective cover around the pesticide with release of the pesticide occurring gradually. U.S. Pat. No. 3,343,941 discloses physically admixing herbicidally active alkyd resins with other herbicides for slow release both by hydrolysis of the chemically incorporated herbicide as well as by physical release of the admixed herbicide. Canadian Pat. No. 846,785 discloses the incorporation of a pesticide in a thermoplastic polymer having desired permeability to the pesticide with release of the pesticide by diffusion of the pesticide through the polymer lattice.
The rate of release and length of protection afforded by the pesticide compositions disclosed in the above patents has not been commercially advantageous in many instances. The pesticide compositions disclosed in Canadian Pat. No. 846,785 release the pesticide rapidly at first and then too slowly to be effective for the purposes desired. Up to as much as 80 percent of the pesticide remains in the polymer after a reasonable period of time. Thereafter, the pesticide is released at too slow a rate to be effective. The rate of release of compositions disclosed in Canadian Pat. No. 846,785 is determined by the difference in pesticide concentration in the soil adjacent the polymer and that in the polymer surfaces exposed to the environment. Pesticide concentration sharply falls over a period of time, slowing the rate of release of the pesticide.
It has remained a problem to devise controlled release biologically active material compositions providing a release of a biologically active material over extended periods of time at effective rates for control. This application is directed to means providing release of the biologically active materials over an extended period of time and preventing the reduction of the release rate of the biologically active material from a biologically active material-polymer admixture below that necessary to be effective by continually exposing fresh surfaces of the polymer-biologically active material mixture to the environment.